This invention relates generally to two-way radio communication systems. More particularly, the invention relates to a system having a single central controller which controls requests for assignment to a limited number of channels to maximize spectral efficiency by channel reuse according to one aspect of the present invention, and which controls requests for assignment to a limited number of channels to reduce adjacent channel interference to and from the requestor by arranging channel assignments according to progressively larger or smaller values of received signal strength according to a second aspect of the present invention.
Trunked communications systems are well known in both the telephone and mobile radio communications art. In such systems, a plurality of subscribers share a limited number of radio transmission channels, or frequencies. These channels are assigned to subscribers such as mobiles or portables by a central controller. The central controller may be implemented as a base control station having central or distributed control logic. The central controller in a trunking system may not only set up and supervise calls, but may discontinue a particular channel assignment by truncating the channel assignment on the conclusion of either the end of a transmission for a transmission trunked signal, or the end of an entire message containing multiple transmissions for message trunked signals. Examples of message trunked signals include interconnect calls, such as a subscriber to land line or land line to subscriber telephone call, or a subscriber accessing a central computer located at the central site or accessible one-on-one via land lines. An example of a transmission trunked communication system is described in U.S. Pat. No. 4,012,597 issued Nov. 24, 1975 to Lynk, Jr. et al., which was assigned to the same assignee as the present invention.
In a trunked communication system, each of the plurality of subscriber radios may be a single as well as a multi-channel unit specially equipped with control circuitry to enable requesting and receiving a channel assignment and to enable operating on the channel assigned by the central controller.
Trunked communication systems provide advantages over well known, conventional, single channel systems, such as those having a dedicate repeater. System congestion problems on a single channel are avoided by the use of a multi-channel system. Additionally, the trunking technique further alleviates congestion problems for a given single channel by allowing other subscribers to utilize the radio channel in between messages or in between transmissions of other subscribers. However, even these forms of trunking systems are limited in the number and variety of calls that may be in progress at any one time. Moreover, message trunked signals, such as interconnect calls, tie up valuable channels for a considerable length of time and at the expense of tranmissions trunked signals on the trunking system.
Thus, there exists a need for an improved multi-frequency, trunked, two-way radio communication system in which a plurality of subscribers are provided access and in which two different types of information signals may operate simultaneously without tying up the valuable and limited number of radio frequency channels. Furthermore, in trunked, two-way radio communication systems heavily loaded with a limited number of channels, there exists a need for a way to judiciously assign a requester to a channel to insure that this requester will not experience interference from users on the adjacent two channels and, furthermore, that this requester will not cause interference to those adjacent users.